1. Field of the Invention
The present invention relates to a sheet-discharge apparatus, a sheet processing apparatus, and an image forming apparatus, and more particularly to apparatuses that can discharge a sheet to a sheet-stacking portion while blowing air on a lower surface of the sheet to be discharged, thereby preventing a reduction in sheet alignment properties without reducing productivity.
2. Description of the Related Art
Conventionally, image forming apparatuses such as copiers, laser beam printers, facsimiles, and combination units thereof include a sheet-discharge apparatus that discharges a sheet on which an image is formed onto a stacking tray. Sheet processing apparatuses that process a sheet on which an image is formed also include a sheet-discharge apparatus that discharges a processed sheet onto a stacking tray.
If a leading edge of a sheet that is a downstream edge in a sheet-discharge direction hangs down when the sheet is discharged, the leading edge may be brought into contact with a stacking tray and be rounded, thereby reducing alignment properties. In order to prevent such a reduction in alignment properties, various techniques are proposed for conventional sheet-discharge apparatuses.
For example, in Japanese Patent Application Laid-Open No. 2010-132372, air is blown on a lower surface of a sheet to be discharged to increase sheet alignment properties. FIG. 11 illustrates a sheet-discharge operation of such a conventional sheet-discharge apparatus. A sheet P on which an image is formed is discharged from a sheet-discharge roller 12b, and stacked on a sheet-discharge tray 91. At this time, a leading edge P1 of the sheet P is away from the sheet-discharge roller 12b and thus hangs down to a state shown by a broken line in the drawing. However, even in such a case, a lower fan 32 blows air toward the sheet-discharge tray 91 to prevent the leading edge P1 from hanging down.
When the sheet P is discharged from the sheet-discharge roller 12b, the lower fan 32 stops, and an upper fan 31 placed above the sheet-discharge tray 91 blows air. Thus, a trailing edge P2 (upstream edge) of the sheet P is brought into contact with a reference wall 91a, and then the sheet P falls onto the sheet-discharge tray 91 while being guided by the reference wall 91a, thereby allowing the sheet P to be stacked with alignment properties.
In the conventional sheet-discharge apparatus having such a configuration, a sheet is sometimes discharged with a trailing edge as an upstream edge in a sheet-discharge direction being curved upward. In this case, when a sheet to be stacked is a sheet with a low basis weight that is a mass per unit area (1 m2), that is, a sheet with low stiffness, a discharged sheet is stacked while fitting a stacking tray, or a stacked sheet having already been discharged and stacked on the stacking tray.
However, when the sheet is a sheet with a predetermined basis weight or more, that is, a sheet with a predetermined stiffness or more, a discharged sheet is discharged and stacked without fitting the stacking tray or a stacked sheet on the stacking tray. In this case, air blown from a fan presses a trailing edge of the sheet, thereby reducing sheet alignment properties on the stacking tray. Alternatively, air blown from the fan may raise the trailing edge of the sheet to close a sheet-discharge outlet.